Merge Aurélien Bompard's branch to factor our archiver-specific configurations
into their own configuration files. Reworked by Barry to use the new external
configuration file support, and to make test suite succeed.

Configuration
-------------
* `[passlib]path` configuration variable renamed to `[passlib]configuration`.
* Postfix-specific configurations in the `[mta]` section are moved to a
separate file, named by the `[mta]configuration` variable.
* In the new `postfix.cfg` file, `postfix_map_cmd` is renamed to
`postmap_command`.

Also:
* More Python 2.7-isms, use assertMultiLineEqual() directly.
* Added external_configuration() and load_external() to mailman.config.config
* ConfigLayer does a blanket set of [postfix]postmap_command so subtests
generally won't have to.

* Python 2.7 is not required. Python 2.6 is no longer officially supported.
The code base is now also `python2.7 -3` clean, although there are still
some warnings in 3rd party dependencies. LP: #1073506

Database
--------
* The `ban` table now uses list-ids to cross-reference the mailing list,
since these cannot change even if the mailing list is moved or renamed.

Interfaces
----------
* The `IBanManager` is no longer a global utility. Instead, you adapt an
`IMailingList` to an `IBanManager` to manage the bans for a specific
mailing list. To manage the global bans, adapt ``None``.

* You can now verify and unverify an email address through the REST API.
POST to .../addresses/<email>/verify and .../addresses/<email>/unverify
respectively. The POST data is ignored. It is not an error to verify or
unverify an address more than once, but verifying an already verified
address does not change its `.verified_on` date. (LP: #1054730)